The invention relates to the dressing of grinding wheels, and in particular to the dressing of the side portions and edge faces of grinding wheels.
It is necessary to dress the side portions of grinding wheels used to grind crankshafts and like articles. This is a particularly demanding job, because of the fine tolerances and complex shapes, e.g. curved surfaces, re-entrants and the like. The present industrial method is to side dress the grinding wheel using a single point diamond, working from the table of the grinding machine. This process is very time consuming as the operator has to mount a fixture on the table, and then move the table to start dressing the wheel. The operator normally dresses one side of the wheel, and then stops to make a measurement for the amount that will be required to be removed from the other side portion of the wheel. Once the operator starts to dress the other side portion, he has to stop when he feels that he may be near the intended size (to make sure that the wheel will not be undersized). The process will take from 4 to 8 hours, dependant on the machine and the type of controls which are available. If the operator .does not exercise enough care and patience, he will damage the surface of the wheel, e.g. cause so-called burn of the part, which could mean scrapping the wheel, or make the width of the wheel undersize and out of tolerance.
It is one object of the invention to provide a method and apparatus for use in dressing a grinding wheel, which requires less time, provides a wheel of high accuracy with little or no risk of damage.
According to the invention in one aspect there is provided a method of dressing a grinding wheel by advancing a dressing tool across the surfaces to be dressed, the method comprising first moving the dressing tool across each side portion to remove material to make the wheel of predetermined width and then across the edge of the wheel to form corner radii, wherein the dressing tool is rotated as it is advanced and the speed of advancement across the side surfaces is different from that across the edge portion.
Preferably the dressing tool is caused to travel in successive paths across each side portion, each path starting remote from the circumference of the wheel and finishing adjacent that circumference.
Preferably the method includes the step of moving the tool away from the wheel once it is reached the circumference and then back to the start position for the next path, whereby the tool dresses the wheel in one direction only.
It is a preferred feature of the invention that the speed of movement across a side portion exceeds that across the edge portion.
It is a further preferred feature that the method includes the step of supplying coolant liquid to the dressing tool to pass between the tool and the surface to be dressed.
Preferably the dressing tool is mounted in a holder supporting a power source for rotation of the tool and the coolant liquid is sprayed in a direction so as to cool also the power source.
Preferably the dressing tool is rotated at a speed of about 1000 to 3500 rpm when dressing the side surface. Preferably the rate of rotation is about 1800 rpm when dressing the side surface and about 2200 rpm when dressing the edge portion.
It is a preferred feature that the dressing tool has particles of diamond or like material for the dressing action which particles have a mean particle size selected to avoid burn or glaze of the grinding wheel surface. Preferably the particle size is about 16 to 28 mesh (US sieve size), most preferably 18 to 20. Preferably the particles are present in a density of about 1.8 to about 3 carats per square inch, most preferably about 2.9 carats/sq.in.
Because of the method of the invention the rotary tool may be bigger than usual, i.e. the rotary tool has a diameter of about 100 mm (3.937 inch)
It is a much preferred feature of the invention that the step of controlling the rate of rotation and the speed of travel of the dressing tool by microprocessor control means.
In a variation the method includes the step of shaping the edge portion in addition to forming the radii, say introducing curvature to the edge portion.
In another aspect the invention includes a rotary tool comprising a frame holding at its upper end a servomotor from which depends a spindle, a rotary dressing tool mounted at the free end of the spindle, the tool having particles of diamond or like material for the dressing action which particles have a mean particle size selected to avoid bum.